Yield losses from corn borers

The advent of Bt corn has prompted numerous questions about the damage potential of the European corn borer. Yield losses from corn borers vary depending on hybrid response, environmental influences, and plant stage attacked. Research conducted at Penn State University shows that tunnels in young plants cause the greatest damage. Yield losses per corn borer for four plant stages are: 5.9 percent (10-leaf stage), 5.0 percent (16-leaf stage), 3.2 percent (blister stage), and 2.4 percent (dough stage). If several corn borers attack the same plant, these percentages can be multiplied by the number of insects, resulting in greater losses. Dropped ears, an additional yield loss factor, are not considered in these percentages but obviously would increase the damage potential of this insect.

The percentage yield loss is an interesting concept, but how do these numbers translate into "real" yield losses in Iowa field corn? Starting in 1991, I began a series of tests that measured the yield loss from second-generation corn borers. Eighteen fields in 10 Iowa counties were sprayed with a variety of insecticides using either an airplane or high-clearance ground equipment. These fields were chosen without any indication of the potential for corn borer populations. All insecticides and an unsprayed check were replicated 2-4 times in each field. Grain yields were taken with a combine, measured in a weigh wagon or on scales, and adjusted to 15.5 percent moisture.

Yield differences between the best insecticide-treated plots and the unsprayed check ranged from 0.9 to 32.6 bushels per acre across the 18 fields. The average yield loss from second-generation Euro-pean corn borers was 8.5 bushels per acre. In 16 of the 18 fields (89 percent), yield loss exceeded four bushels per acre (about a $10 per acre loss).

There are two very important factors to consider relative to these yield losses. First, insect-icides rarely provided 100 percent control of corn borers (the average was 49 percent in these tests) so the yield differences I saw represented only part of the actual loss caused by second-generation corn borers. Second, corn borers during the second generation represent half of the damage equation; we also should expect some yield loss from first-generation corn borers.

There is very little information that shows actual yield losses from first-generation corn borers. I have data from one field in western Iowa where several insecticide treatments were applied to measure the impact of both generations during 1996. In this field, the combined damage of both generations of borers was a yield reduction of at least 17.4 bushels (remember that insecticides don't provide 100 percent control so the actual yield loss was greater than observed). The first generation caused 5.7 bushels loss (Warrior/check treatment minus the check/check treatment), with the second generation causing 11.7 bushels loss per acre.

The bottom line is this: European corn borers have the potential to cause serious reductions in yield. They don't always do it, but based on yield data, there is a high probability for yield reductions of at least $10 per acre in Iowa.

Performance of Pounce and Warrior insecticides on first- and second-generation European corn borer in Iowa field corn, 1996.

First generation treatment

Second generation treatment

Bushels/acre

Bushels increase over check

Warrior

Warrior

182.5

17.4

Pounce

Warrior

178.5

13.4

Warrior

Pounce

177.3

12.2

Pounce

Pounce

176.5

11.4

Warrior

check

170.8

5.7

check

check

165.1

-

LSD 0.05

4.0

This article originally appeared on pages 32-33 of the IC-478 (4) -- April 14, 1997 issue.